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Northwest Flow Snowfall (NWFS) Events: GSP Froude Number Study. NWFS Discussion Group Conference Call Harry Gerapetritis and Blair Holloway 11/19/10. Froude Review. Fr = U/(N*H) U: wind speed orthogonal to the barrier N: the Brunt- Vaisala frequency (static stability)

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northwest flow snowfall nwfs events gsp froude number study

Northwest Flow Snowfall (NWFS) Events: GSP Froude Number Study

NWFS Discussion Group Conference Call

Harry Gerapetritis and Blair Holloway

11/19/10

froude review
Froude Review
  • Fr = U/(N*H)
    • U: wind speed orthogonal to the barrier
    • N: the Brunt-Vaisala frequency (static stability)
    • H: height of the barrier
  • Unitless parameter, Fr increases when U increases or N decreases
  • Therefore, stronger flow and/or decreased stability will result in larger Fr values
  • The thinking for NWFS is that when Fr is high, more flow over the mountains, more precipitation generated, more snow
study background
Study Background
  • Fr archived each hour in the edit area to the right
  • 5 outputs include:
    • GFS (FroudeGFS)
    • NAM (FroudeNAM)
    • RUC (FroudeRUC)
    • Local WRF (FroudeWRF)
    • Average of the 4 (Froude)
  • For our purposes these are the inputs to the calculation
    • Barrier height = 3500 ft.
    • Orthogonal wind direction = 320 degrees
    • Averaged within the 850-950 hPa layer
study background1
Study Background
  • Data collected for the 2009-2010 winter season
  • Events identified using Baker Perry’s catalog of events, synoptic classification (Perry et al. 2010)
  • 15 total events
    • 6 pure upslope NWFS
    • 9 transitioned to NWFS
study background2
Study Background
  • Relied heavily on Baker’s data from Flat Springs
  • Most consistent and reliable dataset available
  • Used Baker’s start/end times to denote events and analyze Fr
  • Might result in some bias towards our northern mountains and elevations above 3500 feet
10 11 february 2010 nwfs event
10-11 February 2010 NWFS Event
  • Highest impact event of the season
  • Occurred with NW flow on the backside of a departing surface low
  • Event that GSP issued some Blizzard Warnings
10 11 february 2010 nwfs event1
10-11 February 2010 NWFS Event
  • Warning level accumulations primarily limited to the northern mountains and the Smoky Mountains
  • Perhaps because low level flow was more like 270-290 degrees, not a particularly amplified upper pattern
  • Also, maybe upstream snow cover played a role?
season summary guide
Season Summary Guide
  • Snow/12 Hours – Flat Springs snow per 12 hours
  • SLR – Snow-to-Liquid Ratio from Flat Springs, NC
  • Fr Before – Average hourly Fr in the 24 hour period before the event began
  • Fr During – Average hourly Fr during the event
  • Difference – Change between Fr Before and Fr During
  • Fr_SD– Average hourly Fr standard deviation within the edit area
summary
Summary
  • Season long Fr trace reveals periods of time when Fr is high but there is little or no upslope snow
  • Can have NWFS with low or high Fr but it appears that higher end events have relatively higher Fr values
  • Overall, very small sample size, especially higher end events
  • Flat Springs only had 2 periods that would have met GSP’s new winter storm warning criteria (4”/12 hours)
    • 1/2/10 07-18 UTC – 5.1” (average Fr=1.06), but no other warning level obs
    • 2/10/10 12-00 UTC – 6.4” (average Fr=2.01)
  • Maybe 12 hour Fr average ~2.00 is more indicative of heavy snow (4”/12 hours)?
  • For operational use, consider in conjunction with traditional NWFS parameters to determine event magnitude?
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